VISUAL SYSTEM: STATE OF THE ART 



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Figure 11 Electron micrograph of a rod photoreceptor terminal in the retina of 

 Negaprion brevirostris . Thick arrow points to rod spherule which contains a single 

 synaptic inclusion of invaginating contacts. Thin arrows point to synaptic ribbons. 

 Original magnification 9000X. Rod pedicle is about 8 (im at its widest point. (From 

 unpublished studies by J.L. Cohen.) 



Squalus. They estimated the receptor population at no more than a few 

 thousand cones per square millimeter and 100 or more rods per cone. 



Horizontal Cells— The layer of neurons proximal (i.e., functionally 

 closer to the brain) to the receptors, the internal nuclear layer, contains the 

 perikarya of horizontal, bipolar, and amacrine cells. Perhaps the most strik- 

 ing feature of the elasmobranch retina is the large cell bodies of the horizon- 

 tal cells (Kaneko et al. 1976), up to 200 jiim across in Mustelus (Stell and 

 Witkovsky 1973b). The size of the horizontal cells is conducive to penetra- 

 tion with microelectrodes, and thus the electrical activity of these cells has 

 been investigated, for example by Dowling and Ripps (19716), Kaneko 

 (1971), and Naka and Witkovsky (1972). It is only recently, however, that 

 their neural function has been appreciated. 



Yamada and Ishikawa (1965) were the first investigators to observe 

 elasmobranch horizontal cells under electron optics. They studied the duplex 

 retinas of Mustelus and Dasyatis and recognized two layers of horizontal 

 cells: an external layer (closest to the photoreceptors) with large cuboidal 



